Very little is presently known of the detailed molecular organization of the contractile apparatus in smooth muscle. The general aim of the proposed research is to define some of the structural properties of the myofilaments in order to gain some understanding of the contractile mechanism. Recent study has revealed that the 100 Angstrom Unit-diameter myofilaments of smooth muscle may play an important role since there is a close correlation between the three dimensional organization of these filaments in the muscle fibers and the length-tension relationship of the muscle. Several methods have been developed to isolate and experimentally manipulate these filaments. The specific aims of the research are to employ electron microscopy and low angle x-ray diffraction to analyze and deduce the structure and organization of the 100 A filaments. The long ranges in smooth muscle in an effort to define the contractile apparatus in terms of a molecular model.